ESLint Stylistic (original) (raw)
arrow-parens
Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.
Rule Details
This rule enforces parentheses around arrow function parameters regardless of arity. For example:
js
// Bad
a => {}
// Good
(a) => {}Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.
js
// Bad
if (a => 2) {
}
// Good
if (a >= 2) {
}The rule can also be configured to discourage the use of parens when they are not required:
js
// Bad
(a) => {}
// Good
a => {}Options
This rule has a string option and an object one.
String options are:
"always"(default) requires parens around arguments in all cases."as-needed"enforces no parens where they can be omitted.
Object properties for variants of the "as-needed" option:
"requireForBlockBody": truemodifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).
always
Examples of incorrect code for this rule with the default "always" option:
js
/* eslint @stylistic/arrow-parens: ["error", "always"] */
a
=> {};
a
=> a;
a
=> {'\n'};
a.then(
foo
=> {});
a.then(
foo
=> a);
a(
foo
=> { if (true) {} });Examples of correct code for this rule with the default "always" option:
js
/* eslint @stylistic/arrow-parens: ["error", "always"] */
() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });If Statements
One of the benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:
js
var a = 1;
var b = 2;
// ...
if (a => b) {
console.log('bigger');
} else {
console.log('smaller');
}
// outputs 'bigger', not smaller as expectedThe contents of the if statement is an arrow function, not a comparison.
If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.
js
var a = 1;
var b = 0;
// ...
if ((a) => b) {
console.log('truthy value returned');
} else {
console.log('falsy value returned');
}
// outputs 'truthy value returned'The following is another example of this behavior:
js
var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?f is an arrow function which takes a as an argument and returns the result of b ? c: d.
This should be rewritten like so:
js
var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;as-needed
Examples of incorrect code for this rule with the "as-needed" option:
js
/* eslint @stylistic/arrow-parens: ["error", "as-needed"] */
(
a
) => {};
(
a
) => a;
(
a
) => {'\n'};
a.then((
foo
) => {});
a.then((
foo
) => a);
a((
foo
) => { if (true) {} });
const f = /** @type {number} */(
a
) => a + a;
const g = /* comment */ (
a
) => a + a;
const h = (
a
) /* comment */ => a + a;Examples of correct code for this rule with the "as-needed" option:
js
/* eslint @stylistic/arrow-parens: ["error", "as-needed"] */
() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;
const f = (/** @type {number} */a) => a + a;
const g = (/* comment */ a) => a + a;
const h = (a /* comment */) => a + a;requireForBlockBody
Examples of incorrect code for the { "requireForBlockBody": true } option:
js
/* eslint @stylistic/arrow-parens: [2, "as-needed", { "requireForBlockBody": true }] */
(
a
) => a;
a
=> {};
a
=> {'\n'};
a.map((
x
) => x * x);
a.map(
x
=> {
return x * x;
});
a.then(
foo
=> {});Examples of correct code for the { "requireForBlockBody": true } option:
js
/* eslint @stylistic/arrow-parens: [2, "as-needed", { "requireForBlockBody": true }] */
(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;